Ground electrodes means in this context devices used to connect an electrode line of a power network comprising an HVDC transmission system, via one or more feeder cables, to a conducting medium such a soil or sea water.
HVDC transmission systems usually have DC voltages above 5 kV and a transmitted power above 10 MW.
As compared with alternating current (AC) transmission systems, HVDC transmission systems require only two conductors. At least one of those conductors is executed as an overhead line or a high voltage cable. For bipolar transmission another conductor of the same kind is used under normal operating conditions, but in monopolar transmission, the ground, that is soil and/or sea water, is used as return conductor for the transmitted DC-current. However, ground electrodes are required also in HVDC transmission systems intended for bipolar transmission to transfer unbalance currents and, under operation in monopolar mode, the whole DC current transmitted by the HVDC system.
Depending on the location of the HVDC-system, the ground electrodes can be located in soil or in sea water. Ground electrodes located in sea water usually have certain advantages as compared to ground electrodes located in soil, such as a better personal safety because they are usually not easily accessible to human beings, and their environment offers a good cooling system and a surrounding conducting medium which has a higher specific conductivity than soil located far from the sea.
This application is concerned with sea electrodes, that is electrodes located in sea water and/or in sea water impregnated matter at the sea bottom, such as gravel, sand, clay and mud.
The sea electrode shall transfer the DC current from an electrode line of the HVDC system via one or more feeder cables to the sea water, which in this context is to be regarded as a good conducting homogeneous medium, and to the sea bottom, which has a lower specific conductivity than the water but usually, and in particular in layers close to the water, higher than for soil layers far away from the sea.
For a general description of ground electrodes in connection with HVDC systems, reference is made to for example E. Uhlmann: Power Transmission by Direct Current, Springer Verlag 1975, in particular pages 255-273.
The sea electrodes are--apart from the requirements as to current and resistance--also required to be electrically safe, to have high operational reliability and sufficiently long service life and in addition, they shall not cause any harmful environmental effects.
The grounding resistance has to be low, usually well below one ohm. In particular for sea electrodes an electric field strength in the sea water in the vicinity of the electrode, which will have a harmful influence on the environment and affect fish and possibly also other aqueous organisms in the vicinity of the electrode, shall be less than a desired level, often specified as 1 V/m.
A conventional sea electrode comprises an active part, herein called the electrode body, which is in electric contact with the sea water and with the sea bottom and through which the current is transferred, interconnection cables for internal connection of parts of the electrode body as described below, and additional parts performing purely mechanical functions, such as for instance electrode holders, supports and mechanical protection parts.
In order to reach a sufficiently low grounding resistance, a sea ground electrode usually comprises a large number of sub-electrodes, each sub-electrode being fed from a separate feeder cable, and comprising as active part at least one sub-electrode element. The surface of each sub-electrode element comprises an active part which is in electric contact with the sea water and/or sea water impregnated matter at the sea bottom. In cases where the sub-electrode comprises more than one such element, these elements are connected to each other by interconnection cables. The sub-electrodes are usually arranged in sections. Each such sub-electrode usually also comprises additional elements such as sub-electrode element holders.
Typically, the sub-electrode elements are manufactured in the form of rods, tubular elements, plates or meshes, which makes them easy to manufacture and to mount.
FIG. 1 illustrates schematically an electrical configuration typical for an HVDC transmission system with sea ground electrodes at both ends. An electric alternating current (AC) power network N1 is via a transformer T1 connected to the AC-side of a thyristor converter SR1 and an AC power network N2 is via a transformer T2 connected to the AC-side of a thyristor converter SR2. On the DC-sides of the converters, a cable LO connects one of their respective poles, and the ground return comprises two electrode lines LE1, LE2, two sea electrodes 15 of similar structure, and the sea water and the sea bottom (not shown) between the sea electrodes. The sea electrode at the converter SR1 comprises a plurality of sub-electrodes 16, each of which being connected to the electrode line via a main feeder cable 2. Each sub-electrode comprises a plurality of sub-electrode elements 161, 162, 163, interconnected by interconnection cables 2', 2", 2'" respectively, and mechanical support members (not shown). The electrode body comprises all the sub-electrode elements 161, 162, 163 comprised in all the sub-electrodes connected to the electrode line.
Ground electrodes for HVDC transmission systems, transferring comparatively high currents, operate, however, with a low average current density, and therefore often cover large areas at the sea bottom. However, an inhomogeneous current distribution with high local current densities on the surface of the electrode body, in particular at peripheral parts of the electrode and at feeding points of the sub-electrode elements, results in high local electric field strength in the sea water.
Usually, sea ground electrodes are protected and kept in place by a ballast, either in the form of a separate solid cover or by a layer of gravel.
FIG. 2A shows schematically an example of the physical layout, seen from above, of a prior art electrode for use as an anode, that is delivering current to the sea water. The sub-electrodes 16, each of which are fed by a separate feeder cable 2, are arranged in an array along a curved line, and are mutually separated from each other with substantially equal distances. However, the arrangement of sub-electrodes along a line as illustrated in FIG. 2A, can be executed also separated from each other with unequal distances.
FIG. 2B shows schematically another example of the physical layout, seen from above, of a prior art electrode for use as an anode. The sub-electrodes 16, each of which are fed by a separate feeder cable 2, are arranged in an array along a substantially straight line, and are mutually separated from each other with unequal distances, in such a way that the outermost located subelectrodes are located at a somewhat shorter distance from their neighboring sub-electrodes than are the sub-electrodes located in the center of the electrode.
FIG. 2C shows a known embodiment of a sub-electrode 16, with its active part, an sub-electrode element 161, executed as a flat, elongated mesh, made of a metal with a low dissolution rate, for instance coated titanium, and supported at its long sides by concrete slabs 41. Such a sub-electrode might also be provided with interconnection cables, connecting the feeder cables with different parts of it. Here, the whole surface of the sub-electrode element constitutes the active part of the surface.
FIG. 2D shows another known embodiment of a sub-electrode, comprising two cylinder-shaped sub-electrode elements 161, 162, connected in parallel by an interconnection cable 2' and fed from a common feeder cable 2. The sub-electrode elements are mounted in a box 42 of concrete for mechanical protection, the box being provided with pass ways (not indicated in the figure) for letting the sea water into contact with the sub-electrode elements.
The sub-electrode according to FIG. 2C can for instance be comprised in an electrode according to FIG. 2A and a sub-electrode according to FIG. 2D in an electrode according to FIG. 2B.
FIGS. 3A-3D show various prior art embodiments of sub-electrode elements.
FIG. 3A shows a sub-electrode element 161 of cylindrical shape, with the feeder cable 2 connected to the cross-section area at one of the ends. Here, the envelope surface S of the sub-electrode element constitutes the active part of the surface of the sub-electrode element, while the two cross-section areas S1, S2 (of which only S1 is shown) of it usually are covered by a non-conducting material and thus are not an active part of the surface of the sub-electrode element.
FIG. 3B shows a sub-electrode element 161 of flat shape, with the feeder cable 2 connected to the mid-point of the element and FIG. 3C shows a sub-electrode element 161 of flat mesh-type shape, with the feeder cable 2 connected to the mid-point of the element. Here, the whole surface of the sub-electrode element constitutes the active part of the surface.
FIG. 3D shows a sub-electrode element 161 in the shape of a bent cylinder, with two feeder cables 2a, 2b, for connection, one of them to each end of the element. The sub-electrode element has a radius R of curvature as from a center of curvature C. The active part S of the surface of the sub-electrode element is in this case the whole surface of it less the two cross-section surfaces S1, S2.
The sub-electrode elements according to FIGS. 3A and 3D can alternatively be of tubular form.
Sea electrodes in cathodic operation, that is where the current flows from the sea water into the electrode body, usually have an electrode body executed in the form of loops of wires or bars of copper.
Usually, it is more difficult to provide such electrodes with mechanical protection than it is with the electrodes intended for anodic operation, as described above, and typically they will also exhibit high current densities and consequently, high electric field strengths in the sea water in the vicinity of the electrode.
One disadvantage with the known sea electrodes is that they typically have a high local electric field strength, usually at the peripheral parts of their surface and at the locations where the feeder cables are connected. As already mentioned, the electric field strength may have a harmful effect on fish and possibly also other aqueous organisms in the vicinity of the electrode.
Another disadvantage with the known sea electrodes is that they usually are designed to operate with a low average current density on the surface of the electrode body in order to keep the maximum electric field strength at a desired value, typically in the order of 1 A/m.sup.2. At least some materials, which can be used as anodes as well as cathodes, thus in contrary to the above-mentioned coated titanium, usable only in anodic operation, and copper, usable only in cathodic operation, typically allow for at least a hundred times higher current density. One consequence is that the electrodes are poorly utilized and another is that they often occupy large areas, and thus may influence local conditions in the sea, such as for instance water streams.
Still another disadvantage with the known sea electrodes is that the ballast executed as a layer of gravel can under certain circumstances, such as strong waves or anchoring of large ships, be destroyed, leaving the electrode unprotected or even damaged and difficult to repair. This is the reason why such electrode body materials as magnetite, graphite and silicon iron, which are mechanically brittle or fragile, have a very restricted use for sea electrodes.